The present invention relates to a photodiode to be used for optical communication. More specifically, the invention relates to a superlattice avalanche photodiode (SLAPD) to be used for a multi Gb/s transmission system, furthermore an avalanche photodiode having a particularly modified superlattice structure.
As s photodiode to be used for a Gb/s band optical communication system, an avalanche photodiode (hereinafter abbreviated to SLAPD) comprising a multiplication layer having a superlattice structure is now under development from a view point of wide bandwidth and low noise characteristics. A superlattice used for SLAPDs comprises a crystal layer wherein lattice are matched on a crystalline substrate. The typical structure thereof is that the well layer thickness (Lw) is 20 to 50 nm, the barrier layer thickness (Lb) is 20 to 50 nm, and the total film thickness (Lt) of the superlattice layer is about 1 .mu.m. Kagawa and others report that characteristics of a SLAPD of the above structure are that the gain-bandwidth product (GB product) is 50 GHz, the ionization rate ratio k is 10 (when the multiplication factor M is 10). (T. Kagawa, et al; Third Optoelectronics Conference (OEC 1990), Technical Digest, 13A2-7, pp. 194 and 195, July 1990, Makuhari Messe)
Avalanche photodiodes and quantum well structures are mentioned, for example, in Japanese Patent Laid-Open Nos. 60-160191, 61-224469, 63-232377, and 3-16276.
To apply the superlattice avalanche photodiode SLAPD to future high speed optical communication systems, it is desirable to increase the gain-bandwidth product (GB product more than 100 GHz when the multiplication factor M is 10) much more with the high ionization rate ratio k (for example, k is 5 to 10 when the multiplication factor M is 10) kept. When the ionization rate ratio k is low, noise increases and the response speed slows down. In a high speed optical signal receiver, the noise power of the receiver increases because the band of the preamplifier becomes wider and it is necessary to increase the signal multiplication factor so as to obtain a predetermined S/N ratio. Therefore, it is necessary to increase the Gb product to more than 100 GHz. To increase the GB product, it is effective to decrease the total film thickness (Lt) of the superlattice layer. When Lt decreases, however, the electric field necessary to obtain a certain multiplication increases and the ionization rate ratio k decreases resultantly.
Therefore, it is impossible to improve the aforementioned characteristics by the method of decreasing Lt with the conventional superlattice structure. Furthermore in the SLAPD, positive holes with a heavy effective mass are stored on the heterojunction interface, so that the response characteristics are degraded. Therefore, it is necessary to realize a superlattice structure which produces no storage effect.